Ion-induced charge emission from unpolished surfaces bombarded by an [Emim][BF₄] electrospray plume

Experiments were conducted at the University of Illinois at Urbana Champaign Electric Propulsion Laboratory to study the ion-induced secondary charge emission from surfaces bombarded by an [Emim][BF₄] electrospray plume. The surfaces were unpolished and had oxide and hydrocarbon layers typical of as-received materials used in electrospray experiments. The effect of changing the bias from −85 to +85 V on the target collecting current from the electrospray plume was measured. These data are used to calculate the yield of positive and negative charged species emission from the target due to bombarding cations and anions. The yields of positive charges per incident cation $(γcation+)$⁠, negative charges per incident cation $(γcation−)$⁠, positive charges per incident anion $(γanion+)$⁠, and negative charges per incident anion $(γanion−)$ are measured for eight different industrial materials commonly used in electrospray diagnostics or thruster and spacecraft construction. These yields range from 0 to 1.3 charges per incident ion over a range of emitter voltages from 1.5 to 2.9 kV and largely display linearly increasing trends with increasing emitter voltage. Over this emitter voltage range, $γcation+$ ranges from 0 to 0.55, $γcation−$ ranges from 0 to 0.75, $γanion+$ ranges from 0 to 0.4, and $γanion−$ ranges from 0.3 to 1.3. The data show for the first time positive and negative charges being emitted due to both cation and anion bombardment and suggest that secondary ions sputtered from the surface are important to the secondary charge emission behavior in electrospray plumes.